To look like Odo, of course. I imagine if Laas had made it to the Great Link before Odo, they'd all look like Laas, with that vaguely Varalan forehead of his.

And Odo's face is more of a sketch of a humanoid visage, and the Founder's weren't too keen on reproducing humanoid appearance unless they were deliberately infiltrating the enemy. Odo's look was minimalist, and the Founder's liked it.

Alternatively, the Odo-type appearance of the Founders is their default appearance (perhaps a holdover from when they were Solids, too), and Odo himself just hadn't the experience to hold a more complex form than his default. Hence the Founders look like Founders when they're not mimicking another form, and naturally the young Founder Odo looks like one, too.